1. Field of the Invention
The present invention relates to a channel optical waveguide type optical device, driving method of the same optical device and manufacture of the same optical device which may be applied to switching of optical path such as an optical fiber and to an optical logical circuit and an optical memory and more specifically to an optical device, a matrix type optical device formed by arranging the plural optical devices, driving method of the same optical device and manufacture of the same optical device which may be driven with high response characteristic, low drive voltage, low crosstalk and low insertion loss and assures to maintain the constant characteristic by utilizing the hysteresis characteristic even under the condition that a voltage is not applied.
2. Description of the Related Art
An optical communication network is now being developed, from a point-to-point optical communication which is individually connecting nodes, to the Add_Drop Multiplexing optical communication which is realized between points and moreover to optical communication which is connecting among the plural nodes in direct with optical signal without conversion to an electrical signal. Therefore, development of various kinds of optical parts required for such communication such as optical branching/coupling device, optical multiplexing/demultiplexing device and optical switch or the like is very important and particularly a matrix type optical switch utilizing the plural optical switches (or optical gates) is one of the most important parts among various kinds of optical parts because it is used for switching the plural optical fibers depending on demands or for switching the fibers to assure the alternative route when a failure occurs in the network.
A type of optical switch can be sorted as a bulk type and optical waveguide type switches. A bulk type switch is used to switch the optical path by mechanically driving a prism, mirror or fiber or the like and has the merits in less dependence on wavelength and comparatively low loss but also has problems that switching speed is rather low, reduction in size is difficult, that structure is not suitable for matrix type arrangement and that assembling and adjusting process are rather complicated to result in considerable difficulty in mass production and cost reduction. The optical waveguide type switch has excellent merits in the switching speed, reduction in size and integration and mass production and therefore the optical waveguide type matrix optical switch is widely discussed.
The optical waveguide type matrix optical switch can roughly be sorted to two kinds of mode. In the first mode, a branching type channel optical waveguide for connecting an input port and output port is switched through combination of plural sets of the optical switches or optical gates based on a certain principle. In the second mode, an optical deflector is provided between the input port and output port to deflect the optical beam of input port toward the output port. In these modes, the optical waveguide type optical switch of the first mode is now discussed most widely from the viewpoint of flexibility of design and less amount of loss.
In the optical waveguide type optical switch, a channel optical waveguide is generally formed on LiNbO3, compound semiconductor, quartz or polymer and an optical switch for electrically controlling the traveling direction of optical beam or an optical gate for electrically controlling the switching of traveling of optical beam is provided at the crossing areas of respective optical paths.
The optical waveguide type optical switch utilizing quartz or polymer has a merit that the core size can be set almost equal to the mode field diameter of an optical fiber and insertion loss can be set small because optical coupling efficiency from the optical fiber is good, but also has a problem that a response rate is rather slow because a current is applied to a heater provided at the surface of optical waveguide in order to switch the traveling direction of optical beam by changing refractive index due to the thermoxe2x80x94optical effect. For example, it is disclosed in the J. Lightwave Tech. 16(1998) 1049 by T. Watanabe et al. That the response rate is extended up to 6 ms. Moreover, this optical switch also has a problem that only one electrode requires the power consumption as much as several hundreds of mW and its application field is limited because the heating system by heater is employed.
In addition, an optical waveguide type optical switch formed of an organic non-linear optical material is also proposed. In general, the optical waveguide can be formed of an organic non-linear optical material by doping organic non-linear molecule into the polymer or by giving the non-linear structure to the side chain or main chain portion of polymer to attain the electric field alignment polymer to which poling has been conducted through application of electric field. However, as disclosed in the reference, O plus E, 186(1995), 98 by Kainoh, the electric field alignment polymer has a problem of temperature stability in comparison with the oxide ferroelectric material and development of an element which may be enough for practical use is not yet insufficient in current.
In the case of the optical waveguide type switch utilizing a compound semiconductor or quantum well, high speed response can be realized but it also has a problem, as disclosed in the reference Optoelectronics 24 (1995) 324 by Koizumi, that a core size is small and optical coupling efficiency from an optical fiber is rather bad and thereby insertion loss becomes large. Accordingly, various effects have been made for this optical waveguide type optical switch. Moreover, this optical switch has the problems that the switching characteristic is deteriorated because optical absorption is generated simultaneously with the switching by the electrical field application and that it is difficult to form a large size matrix optical switch because the wafer size is limited.
In the case of the LiNbO3which is the most typical optical switch material and is one of the oxide ferroelectric material, optical condition changes at a high speed when a voltage is applied to the electrode of optical switch because the refractive index changes due to the electro-optic effect and the traveling direction of optical beam changes depending on the setting of optical condition. Therefore, each optical switch is capable of selectively outputting the light beam from a couple of input ends to a couple of output ends. Accordingly, when the traveling direction of the optical beam is adequately set in each stage, the light beam from the input port can be sent to the desired output port.
Moreover, in an optical switch using LiNbO3, a Ti diffusion type optical waveguide and a proton exchange type optical waveguide is formed to a single crystal wafer, but since the core size may be determined equal to the mode field diameter of an optical fiber and optical coupling efficiency from optical fiber is good, insertion loss becomes small. Accordingly, this optical switch is known as the optical switch for the level of practical use.
However, since a voltage is applied in the structure by providing a coplanar type electrode to the surface of optical waveguide, the distance between electrodes becomes long and an ideal field profile cannot be attained and the drive voltage as high as 40 volts is required in order to eliminate dependence on polarization disclosed, for example, in the Electron. Lett. 29 (1993) 765 by H. Okayama and M. Kawahara. Moreover, since the optical waveguide is formed by the Ti diffusion and proton exchange on the single crystal wafer, an effective refractive index of the channel optical waveguide cannot sufficiently be set higher than that of the peripheral area and therefore the difference of the refractive index cannot be set large. Accordingly, it is also required to assure the radius of curvature of S-shape channel optical waveguide as large as 50 mm and the size of the 8xc3x978 matrix optical switch becomes as large as about 70 mm in the example by H. Okayama et. al.
The principle for switching the optical path of these optical switches can be represented by the method for controlling the optical path by applying an electric field to a directional coupler in which two optical waveguides are provided approximately, the Mach-Zehnder type method for isolating an input beam to a couple of beams with the directional coupler to give a phase difference to the light beams passing respective light paths with the refractive indices generated by the electric field and for switching the output end by controlling the interference condition in the directional coupler in the output side, the method for switching the optical path by controlling the interference between optical modes at the X crossing portion, the digital type method for switching the optical path by controlling distribution in the lateral direction of optical mode with the refractive index generated by the electric field in the Y branching portion or asymmetrical X crossing portion, and the method for switching the optical path through total reflection or Bragg reflection by controlling the refractive index through provision of the electrode at the X crossing portion. These methods are disclosed, for example, in the Electron. Lett. 29 (1993) 765 by H. Okayama and M. Kawahara, the Japanese Published Unexamined Patent Application No. Hei 7-318986 and the Japanese Published Examined Patent Application No. Hei 6-5350 or the like.
A digital type optical switch among these optical switches is called as a digital type because after the optical path is switched by a constant voltage, this condition is maintained even if higher voltage applied and plural operating points are never generated and this digital type optical switch is attracting particular attention because of the merits that it has excellent tolerance of operating voltage, that dependence on polarization can be eliminated and that dependence on wavelength is rather small. However, this digital type optical switch requires higher drive voltage or longer electrode length in comparison with the other optical switches.
In addition, as illustrated in FIG. 15, the Y branching type channel waveguide of the digital type optical switch is branched at the acute angle portion to form a branching portion 1. In the case of ideal shape, two channel waveguides are crossing in the angle under 1xc2x0 and the width between channels becomes gradually narrow at the branching area 1 until it becomes zero. However, it is difficult in the patterning process by the photolithography to form the ideal shape because of the limitation of resolution and therefore the shape has to be dulled, as illustrated in FIG. 16, in which the end part of the acute angle portion is cut at the area where the distance between channels of the branching portion 1 becomes about 1.5 xcexcm. Such deviation from the ideal shape does not cause serious influence on loss but causes serious influence on crosstalk. In the case where the opening angle of the Y branching portion is set to 0.5xc2x0 and the refractive index of one branching waveguide is lowered by about 0.0008 through the electro-optic effect, crosstalk may be set to 20 dB or less in the ideal shape of FIG. 15 but in the case of the shape of FIG. 16, crosstalk is deteriorated up to about 12 dB and a larger change of refractive index, namely a large voltage application is required to attain the crosstalk of 20 dB.
Therefore, the reference, Electron. Lett., 32 (1996) 544 by R. Moosburger et al. proposes, as illustrated in FIG. 17 and FIG. 18, the method of manufacturing a digital type optical switch having the ideal shape by using a photomask 2 in which the acute angle portion of the branching portion 1 of the channel waveguide is dulled through utilization of the under etching effect during the wet etching of the Si substrate 3. However, this method has a problem that the under etching width becomes almost equal to the etching depth because the isotropic etching is generally performed and the channel depth cannot independently controlled and therefore the manufacturing tolerance becomes insufficient. Moreover, this method also has a large difficulty other than the manufacturing tolerance because it is known that etching is never performed easily in the oxide ferroelectric material which is a typical optical switch material. In addition, a problem on the manufacturing is also occurring not only in the branching portion of the Y branching type channel waveguide but also in the branching portion of the asymmetrical X crossing type channel waveguide.
As explained above, even in the case of using any material of LiNbO3, compound semiconductor, quartz or polymer, it has been impossible to attain an optical waveguide type matrix optical switch which simultaneously satisfies the conditions of optical switch size, drive voltage (or drive current or power consumption), switching speed, crosstalk, insertion loss and temperature stability.
Moreover, a branching type optical switch of the related art, in which a channel optical waveguide is formed to LiNbO3, compound semiconductor, quartz or polymer and an optical switch for electrically controlling the traveling direction of a light beam or an optical gate for electrically controlling the traveling of a light beam by the switching operation is provided at the crossing portion of each path, is always requested to continuously apply a voltage or current to maintain the switching condition and therefore an optical device having the memory effect has been desired from the point of view of new application field such as reduction of power consumption and optical logical circuit or the like.
The present invention has been proposed to overcome the problems of the related art explained above and it is therefore intended to provide an optical device which assures high speed drive, excellent temperature stability, low drive voltage and low crosstalk and insertion loss.
The present invention also provides a compact size optical device and a matrix type optical device which can use a large number of optical devices.
The present invention also provides a driving method of an optical device for obtaining the stable driving characteristic without any hysteresis and a driving method of an optical device for driving the device by storing the predetermined condition using the hysteresis.
The present invention also provides a method of manufacturing an optical device with good accuracy.
The inventors of the present invention has completed, as the result of investigation, the present invention by finding out the problems of the related art can be solved by the optical devices to be explained below.
The optical device of the present invention includes an epitaxial or single orientation optical waveguide layer which is formed of an oxide ferroelectric material having an electro-optic effect and allows formation of a channel optical waveguide and electrodes having an upper electrode and a lower electrode to apply a voltage to the channel optical waveguide.
Owing to this structure, since a voltage is applied to the channel optical waveguide from the upper and lower directions, the voltage can be applied more effectively in comparison with the case where the voltage is applied from the adjacent electrodes at the upper area of the channel optical waveguide of the related art.
The other optical device of the present invention is characterized in that an oxide ferroelectric material is a non-linear material which shows hysteresis in the change characteristic of refractive index for a voltage and has different characteristics when the voltage is reduced to zero after the single polarity voltage is applied to the channel optical waveguide and when the voltage is reduced to zero after the voltage of the inverse polarity to the single polarity voltage is applied to the channel optical waveguide.
The characteristic can be maintained in the drive of the optical device between the channels even when the optical device is isolated from the power source by giving different characteristics to the channels under the zero voltage condition by utilizing the hysteresis characteristic. Therefore, not only application as a memory using such function is enabled but also power consumption required for drive of the optical device can be remarkably reduced.
Moreover, the other optical device of the present invention includes a single crystal substrate which is used as a conductive or semiconductive lower electrode or a single crystal substrate on whose surface a conductive or semiconductive thin film is provided as a lower electrode, an epitaxial or single orientation buffer layer formed of an oxide provided on the single crystal substrate, an epitaxial or single orientation optical waveguide layer formed of an oxide ferroelectric material having an electro-optic effect which is provided on the buffer layer and has formed a channel optical waveguide of which an optical path is switched when a voltage is applied to a branching portion of an optical path for incident light and an optical path for outgoing light and a couple of upper electrodes for applying a voltage to the branching portion of the channel optical waveguide.
With the arrangement explained above, it is possible to introduce the structure that the optical waveguide formed of an oxide ferroelectric material is sandwiched by the upper and lower electrodes to realize a low drive voltage without deteriorating the low optical transmission loss characteristic and thereby it is also possible to form a small size optical device by remarkably shortening the length of each electrode required to form the optical device through reduction of low drive voltage.
Moreover, since an optical switch is manufactured by growth of a buffer layer, an optical waveguide layer and a clad layer on the substrate, the refractive indices and thicknesses of the substrate, buffer layer, optical waveguide layer and clad layer can be selected respectively unlike those of the existing material. For instance, when Pb(ZrxTi1xe2x88x92x)O3 (0 less than xc3x97 less than 1.0) is used as the buffer layer, optical waveguide layer and clad layer, a refractive index for the wavelength of 0.633 xcexcm can be selected depending on the composition for the wider range from about 2.45 to 2.70 and mutual crystal matching is also excellent.
The other optical device of the present invention is characterized in that the optical waveguide layer has a refractive index which is larger than that of the buffer layer.
When transparency of a conductive substrate is low, if a light beam is guided to the optical waveguide provided on the substrate, a part of total light intensity penetrates to the substrate and the element penetrated to the substrate is absorbed because the transparency of substrate is low and transmission loss is generated by transmission of optical beam. However, as illustrated in FIG. 19, if the area in which the light beam is penetrated is replaced with a buffer layer 4 having the refractive index which is lower than that of the optical waveguide layer material, the buffer layer 4 functions as an isolating layer for isolating the optical waveguide layer 6 and conductive substrate 3. Thereby, the light beam is never absorbed by the conductive substrate 3 and transmission loss can therefore be reduced.
The other optical device of the present invention is characterized that a clad layer which is formed of an oxide and has a refractive index which is smaller than that of the optical waveguide layer is provided between the optical waveguide layer and upper electrode.
When an upper metal electrode is provided on the optical waveguide layer, if the lightwave frequency in the optical waveguide layer exceeds the plasma frequency of a metal electrode, the element penetrating to the metal electrode due to the optical transmission is intensively absorbed by the carrier in the metal, resulting in transmission loss. However, as illustrated in FIG. 19, when the area where the light beam is penetrated is replaced with a clad layer 7 having the refractive index which is lower than that of the optical waveguide layer material, the clad layer 7 functions as an isolating layer for isolating the optical waveguide layer 5 and metal electrode 8. Thereby the light beam is never absorbed by the upper metal electrode and transmission loss can be reduced.
The other optical device of the present invention is characterized in that this device is a digital type optical switch in which the channel optical waveguide is an X branching type optical waveguide and the upper electrode is a digital type switch provided to control switching of the optical path with a voltage applied to the branching portion.
The digital type optical device has excellent merits that after the optical path is switched with a constant voltage, that this condition is maintained even if a higher voltage is applied and the plural operating points are never generated, that tolerance of operating voltage is excellent, and that dependence on polarization can be eliminated and dependence on wavelength is rather small, etc.
The other optical device of the present invention is characterized in that the device is formed as a total reflection type switch or Bragg reflection type switch wherein the channel optical waveguide is the X crossing type optical waveguide and the upper electrode is provided to control the switching of the optical path with application of a voltage to the branching portion.
The matrix optical device of the present invention is characterized in that the branching portions of the optical devices are arranged in the form of a matrix on the same substrate and the optical path is switched from a desired optical path for incident light among the plural output optical paths to a desired input optical path among the plural input paths for outgoing light.
Since the ports having the adequate interval can be switched with a low drive voltage and the electrode length and bending channel optical waveguide length can also be shorted as required, a larger scale matrix optical device can be formed on the substrate wafer of the identical size as the existing substrate.
The driving method of an optical device of the present invention is a method for driving the optical device explained above and is characterized in that after the single pole initial voltage is applied to the channel optical waveguide from the upper electrode, and a voltage of the same polarity as the initial voltage is applied for the driving purpose.
By applying such an initial voltage, operation of optical device in which hysteresis is never generated can be realized.
The other optical device driving method of the present invention is further characterized in that the initial voltage which is enough to give electric field higher than the coercive electric field is applied to the channel optical waveguide.
Particularly, when the initial voltage which gives the electric field higher than the coercive electric field is applied, the hysteresis characteristic can be eliminated almost perfectly.
The other optical device driving method of the present invention is a method for driving the optical device explained above and is characterized in that the drive is driven by utilizing the first characteristic obtained when a single polar voltage is applied to the channel optical waveguide and this voltage is thereafter reduced to zero and the second characteristic different from the first characteristic when a voltage of inverse polarity inverted to the single polar voltage is applied to the channel optical waveguide and this voltage is thereafter reduced to zero.
In the other optical device driving method of the present invention, even if a voltage is not applied, the conditions which are required for driving the optical device may be maintained by giving the first characteristic to one channel and the second characteristic to the other channel.
The manufacturing method of an optical device of the present invention is a method for manufacturing an optical device including an epitaxial or single orientation optical waveguide layer which is formed on an oxide ferroelectric material having an electro-optic effect and has formed a channel optical waveguide and an electrode having an upper and a lower electrodes for applying a voltage to the channel optical waveguide. This manufacturing method is characterized in that the optical waveguide layer is formed through solid phase epitaxial growth by heating an amorphous thin film.
In the case of the patterning of a polycrystal thin film by a similar method, an edge, a side wall and a surface are formed roughly due to the uneven areas by random crystal grains but in the case of using solid phase epitaxial growth, the patterned amorphous thin film is formed by solid phase epitaxial growth and therefore the channel optical waveguide having smooth side wall and surface and showing less amount of optical loss due to the scattering can easily be formed.
The other optical device manufacturing method of the present invention is an optical device manufacturing method explained above and is characterized in that the etching is wet etching.
In the patterning process by the photolithography, it is difficult to form the ideal shape, from the limitation of resolution, for the acute angle portion of the crossing portions of the channel waveguide at the Y branching or asymmetrical X crossing portion and here rises a problem that the crosstalk is deteriorated. However, an ideal shape can be formed with good control ability and accordingly the crosstalk can be improved by combining solid phase epitaxial growth with wet etching.
The other optical device manufacturing method of the present invention is an optical device manufacturing method explained above and is characterized in that the amorphous thin film is formed by coating of an organic metal compound solution. Solid phase epitaxial growth utilizing coating and of organic metal compound solution and heat treatment thereof provides the effect for flattening the surface including the level different portion.
The other optical device manufacturing method of the present invention is an optical device manufacturing method explained above and is characterized in manufacturing an optical device by applying, to the formed channel optical waveguide, an initial voltage in such an amplitude as giving the electric field higher than the coercive electric field.
When the initial voltage which is enough for giving the electric field higher than the coercive electric field is applied, the changing characteristic of the refractive index of the oxide ferroelectric material for the applied voltage does not show any hysteresis and this character becomes the linear characteristic and thereby an optical device which assures stable drive can be manufactured.
In the other optical device manufacturing method of the present invention, after an amorphous buffer layer having an amorphous thin film is formed, a part which becomes the channel optical waveguide of the amorphous buffer layer is removed by etching process, thereafter an epitaxial or single orientation buffer layer is formed through solid phase epitaxial growth by heating the amorphous buffer layer, an amorphous optical waveguide layer having the amorphous thin film of an oxide ferroelectric material having the electro-optic effect is formed on an epitaxial or single orientation buffer layer and the epitaxial or single orientation optical waveguide layer is formed through solid phase epitaxial growth by heating the amorphous optical waveguide layer.
Thereby, a channel waveguide having higher accuracy may be formed with higher productivity even when the oxide ferroelectric material is used for which there is no effective method to form the conventional projected or recessed optical waveguide.
Here, on the occasion of forming the channel waveguide by etching the optical waveguide layer using the oxide ferroelectric material, the optical waveguide is formed with an amorphous thin film and it is then etched to form the channel waveguide. Therefore, higher accuracy and higher productivity can be realized even in the case of solid phase epitaxial growth.
The other optical device of the present invention is characterized in that it is provided with an epitaxial or single orientation optical waveguide layer formed of an oxide ferroelectric material having an electro-optic effect and has formed a channel optical waveguide and an electrode for applying a voltage to the channel optical waveguide wherein the oxide ferroelectric material is a non-linear material which shows a hysteresis characteristic in its changing of the refractive index for the voltage and has different characteristics when a single polarity voltage is applied to the channel optical waveguide and the voltage is thereafter reduced to zero and when a voltage of inverse polarity to the single polarity voltage is applied to the channel optical waveguide and thereafter it is reduced to zero. Therefore, a change of the refractive index can be stored even under the non-electric field condition and accordingly an optical device which may be used as a new type memory device with lower power consumption can be obtained.
In the present invention, xe2x80x9csingle orientationxe2x80x9d means the characteristic in which crystal orientation is aligned in one direction at least by the xcex8xe2x88x922xcex8X-ray diffraction pattern, namely the random orientation plane is identified as 1% or less of the diffraction intensity of the single orientation plane.